The present invention relates in general to a system and method for communicating information between distributed stations. More particularly, the present invention relates to a timed token loop which provides three or more classes of service with a priority relationship between classes. The three classes of service provided allow guaranteed bandwidth, interactive and batch services. These classes are implemented by timing the rotation time of a write token to measure instantaneous load and limiting transmission of information by class of service and the observed write token rotation time.
The synthesis of the architecture for future office automation systems will be effected to a large extent by the available communication mechanisms supporting those systems. Circuit switched communication as provided by PBXs is efficient for handling voice traffic, but is limited by the maximum bandwidth available for data traffic. The inefficiencies inherent in using circuit switched serice for communication of bursty data traffic are being solved for low data rate devices through submultiplexing in the PBX. In contrast, local area networks have been optimized for efficient transmission of data at high burst rates with little consideration for handling of digital voice.
Broadband systems provide the capability for carrying both voice and data over the same media through separate logical networks for voice and data on the same physical media. This hybrid approach solves some problems by using a single media, but does not allow the flexibility given by integration of data on the same logical network.
Distributed communication systems based on token loop structures are well known in the prior art. Thus, in the article by David J. Farber titled "A Ring Network", Datamation, February 1975, pp. 44-46, a collection of minicomputers connected by a ring-like digital communication system is disclosed. Similarly, the work by Newhall and Farmer on token controlled rings is exemplified in a paper titled "An Experimental Distributed Switching System To Handle Bursty Computer Traffic", in Proc. ACM Symp. Problems in the Optimization of Data Communications Systems (Pine Mountain, GA, Oct. 1969), pp. 31-34.
The work of both Newhall and Farber was primarily applicable to data type traffic where all stations were given an essentially equal opportunity to transmit. Thus the drawback of both systems is that they cannot support a guarantee of bandwidth.
The inability to guarantee bandwidth is also a drawback of the Ethernet system and all CSMACD-type protocols for local area networks.
The Cambridge ring, which is based on the work of Pierce, is more adapted to a circuit type of service. In such a system, you essentially have a boxcar (which corresponds to available bandwidth) into which data may be inputted or extracted. A fixed allocation procedure may be implemented which reserves the boxcar for a station to use. The drawback of the Cambridge ring is that there's no simple or obvious way to use the boxcar when its not being used by the station that has a fixed reservation on it.
It is the general object of the present invention to overcome these and other drawbacks of the prior art by providing a timed token protocol method and apparatus which integrates the favorable characteristics of both circuit switched and packet switched communications and provides these characteristics to a station through a single physical and logical interface.
It is another object of the present invention to provide a protocol adaptable to either a physical or logical loop which provides three or more classes of service with a priority relationship between classes, the three classes allowing guaranteed bandwidth, interactive and batch services.
It is still another object of the present invention to provide a timed token protocol for a loop communications network which allows the bandwidth guaranteed to a first station to be used by that first station when needed and further allows the same guaranteed bandwidth to be used by another station when it is not needed by the first station.
It is yet another object of the present invention to provide a token controlled loop which provides efficient utilization of available bandwidth by the stations configured on the loop.
It is an additional object of the present invention to provide a timed token protocol adaptable to either a physical or logical loop which provides for stations configured on the loop, each station being of one or more of the multiple classes of priority.
It is a further object of the present invention to provide a timed token protocol for stations interconnected in a continuous loop structure, the protocol providing for stations with different priority levels, wherein stations of the highest priority level are capable of allocating a guaranteed minimum bandwidth to themselves to the exclusion of other stations of the same priority.
It is still a further object of the present invention to provide a timed token protocol for stations interconnected in a continuous loop structure, the protocol providing for stations with different priority levels, wherein stations not of the highest but of equal priority level are guaranteed a pool of bandwidth which may be fairly shared between them.
These and other objects, features and advantages of the present invention will become more apparent from the detailed description of the preferred embodiment when read in conjunction with the drawings.